The power network is, in particular, a low-voltage network of an industrial plant by which a three-phase alternating current is routed to a load. The load is in particular an electrical machine, for example an electric motor.
In such an electrical plant, equipment influencing active power is conventionally used for switching, controlling and/or regulating the electric current. These may be e.g. mechanical and/or electronic switching devices to operate induction motors or resistive loads, but also inverters for variable-speed operation of motors. These can also be e.g. inverters for feeding re-generatively generated power into the power network.
For this purpose, the equipment influencing active power is connected by its input-side connection points to lines of the power network and connected by way of its output-side connection points to lines of the electrical load or, as the case may be, a power-generating unit. During operation of the plant, short-circuits may occur, if only very rarely as a rule, between the lines or within the connected load or power-generating unit.
The currents flowing in the case of a short-circuit can be very high and without additional measures can cause destruction of the equipment influencing active power, the lines and the load or power-generating units.
To prevent endangerment, associated with the destruction of plant parts, of human health and environment, appropriate overcurrent protective devices are conventionally connected upstream of the equipment influencing active power, and these switch off the short-circuit current sufficiently quickly. An overcurrent protective device is by way of example a circuit breaker. If a circuit breaker detects a short-circuit current at the power network it can interrupt the power supply for equipment connected downstream of the circuit breaker via the supply line.
The very high currents produced in the case of a short-circuit flow from the generator via the power network to the short-circuit point and are switched off very quickly by the overcurrent protective device. All lines and equipment via which the short-circuit current flows are loaded excessively heavily and partially destructively for the duration of the short-circuit. The destruction within equipment also results on circuit boards, if the short-circuit current flows via them, by the vaporization of the conductor traces or parts of components and the resulting arc and the conductive plasma.
The level of the short-circuit current is determined by the impedance of the circuit and the phase angle at the moment of its creation. The affected equipment will be destroyed depending on the short-circuit current.
In the field of industrial automation engineering care is therefore taken in the development of plant that, at the place of installation of the equipment, the maximum possible short-circuit current cannot be greater than the prospective short-circuit current with which the equipment has been tested, and that overcurrent protective devices are connected upstream of the equipment, with which this test was performed, or overcurrent protective devices are used which give rise to a lower destructive power in the equipment in the case of this prospective short-circuit current than with the tested overcurrent protective devices. A distinction is made in the choice of overcurrent protective devices between the types of protection.
In the case of protection according to classification 1, the equipment may be defective after the short-circuit but none of the electrically conductive parts of the equipment may be touched. In the case of protection according to classification 2, the equipment is undamaged and can be operated again after replacement or resetting of the overcurrent protective devices.
Since protection according to classification 2 requires expensive overcurrent protective devices, often only the more cost-effective and simple protection according to classification 1 is chosen due to the rarity of short-circuits.
Since the maximum possible level of the short-circuit current at the place of installation of the plant is often not accurately known or must first be calculated in a relative complex manner by the project engineer, project engineers prefer equipment that in the case of the simpler protection according to classification 1, in combination with a conventional overcurrent protective device, allows the highest possible, maximum possible short-circuit current at the place of installation. In this way the normally complex calculations regarding the maximum possible short-circuit current level are unnecessary and the development cost for the plant drops significantly.